EP0081007B1 - Superconducting tunnel junction device - Google Patents
Superconducting tunnel junction device Download PDFInfo
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- EP0081007B1 EP0081007B1 EP81110277A EP81110277A EP0081007B1 EP 0081007 B1 EP0081007 B1 EP 0081007B1 EP 81110277 A EP81110277 A EP 81110277A EP 81110277 A EP81110277 A EP 81110277A EP 0081007 B1 EP0081007 B1 EP 0081007B1
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- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
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- H10N60/00—Superconducting devices
- H10N60/10—Junction-based devices
- H10N60/128—Junction-based devices having three or more electrodes, e.g. transistor-like structures
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- This invention relates to a three terminal superconducting tunnel junction device comprising three conducting electrodes which are separated from one another by two tunnel barriers to form two tunnel junctions. More particular, the invention concerns a superconducting device whose current-voltage characteristic is altered by heavy injection of excess quasi-particles.
- the device can be used for logic operations, or as an amplifier, and has an output that is sufficiently large to drive another such device.
- Superconducting switching devices are well known in the art, and are represented by devices such as the cryotron and the Josephson tunneling device.
- the cryotron In the cryotron, the superconducting gap of a film is caused to vanish (i.e., the film becomes normal). This can occur by several mechanisms, including passing a current through the film having a magnitude greater than the critical current of the film applying an external magnetic field, and by heating the film.
- a Josephson tunneling device is one which exhibits a zero-voltage current between two superconductors separated by a very thin tunnel barrier. By increasing the current through the device, or by applying a magnetic field to the device, the device can be made to switch to a non-zero voltage state. However, the device remains superconducting at all times.
- Another superconducting device is that represented by the transistor of U.S. 4,157,555.
- three superconductors are separated from one another by two insulating layers to form two tunnel junctions.
- a. change can result in the total current flow through one of the junctions, which change is greater than the amount of current injected into one of the superconductors.
- quasi-particles are injected into the middle electrode, but the number of quasi-particles injected is small. That is, the number is in the order of the number of thermally axcited quasi-particles in the superconductor.
- the device of U.S. 4,157,555 cannot function as a switch in the digital sense. Because the superconducting gap remains near its equilibrium value, that device functions only as a small signal analog amplifier (even if the tunnel current density of its output junction is much greater than the tunnel current density of its input junction) and it is not possible to provide a sufficiently large output to drive another similar device. Stated another way, the output of that device is not sufficiently large to change the state of another such device being driven by it.
- the difference in tunnel resistance between the two tunnel junctions of the device is such that the device is restricted to small signal analog amplification.
- recombination times must be very long with respect to the tunneling rate, and for this reason aluminum is chosen as a representative superconductor.
- these materials are not favorable if large signal amplification is desired, or if it is desired to have a switch which has distinguishable output states and which can drive another such switch.
- the transistor of U.S. 4,157,555 is not an inverter and cannot operate as a switch in a digital sense. Its collector junction is biased at an amount less than the gap voltage and for this reason, as well as the small number of quasi-particles injected, a small output results. At the present time, there is no known cryogenic switch which provides digital inversion, as can be obtained with semiconductor bipolar and FET devices.
- the operation of the present device is based on QUasi-particle /njection Tunneling, and hence is called QUITERON. It relies on heavy injection of excess quasi-particles in order to cause a superconducting gap to drastically change, and generally to vanish. It distinguishes from the experiments of Testardi, Phys, Rev. 84, 2189 (1971) in that a complete three terminal device is obtained capable of digital switching. In Testardi's experiments, he discovered that thin superconducting films can be irradiated by optical pulses to drive them into a normal state and back to the superconducting state very rapidly. Testardi suggested that an excess number of quasi-particles larger than the usual thermal equilibrium number were responsible for this phenomenon. However, no devices were suggested or described.
- Non-equilibrium superconductivitVis-â subject of intense investigation as represented by the work of Ginzberg, Phys. Rev. Lett., 8, 204 (1962).
- Ginzberg describes a double junction configuration used to measure quasi-particle recombination life-time.
- K. E. Gray Appl. Phys. Lett., 32, 392 (1978) reported measurement of small signal gain in a double junction configuration and found that, at an extremely low level of injection, its operation was analogous to that of a transistor. This operation is described more fully in the Gray patent previously referenced (U.S. 4,157,555).
- cryogenic switch usable for digital switching and large signal analog amplification.
- This switch uses heavy injection of excess energetic quasi-particles into a thin superconducting film, in order to drastically change the superconducting gap of this film. In most applications, the superconducting gap is driven to zero.
- This device can be used to switch other devices since it provides a large output, and can be used to build a NOR block.
- This NOR block can in turn be used to design all types of logic circuits, without the requirement for separate AND, OR, and INVERT circuits, as would be needed when using other types of cryogenic switches.
- the present device is based upon the discovery of an unexpected threshold power density relationship in which the current-voltage characteristics of the device drastically change when the superconducting gap A changes by a large amount.
- This threshold power density relationship leads to a switch exhibiting high gain, inverting characteristics, high noise immunity, and high discrimination between its output states.
- the superconducting device is comprised of a plurality of conducting layers which function as electrodes, there being tunnel barriers between the electrodes.
- a tunnel junction termed an injector junction
- injector junction is used to heavily inject excess quasi particles into a superconducting layer which is one electrode of a second tunnel junction, termed the acceptor junction. Due to the heavy injection of excess quasi particles, the current-voltage characteristics of the acceptor tunnel junction are changed. In this manner different voltage states are provided which are easily distinguishable from one another.
- This device can provide an inverting characteristic, and is the only known cryogenic device which has this property. Because of this, it can be used to design circuits, such as NOR blocks, which are similar to those designed with bipolar and FET devices.
- the device Since the device is capable of a multiple number of output states which are clearly distinguishable from one another, it can be used as a switch. Due to the large output that results, one such switch can be used to drive another in order to make complete circuits comprised of these switches.
- An additional application is that of an analog amplifier in which analog signals can be greatly amplified. For this use, heavy injection of excess quasi particles is also required.
- the modification of the tunneling properties of the acceptor junction depends on a heavy injection of an excess number of quasi particles.
- the number of quasi-particles injected is greater than the usual thermal equilibrium number of quasi particles, and is typically at least two orders of magnitude greater than the number which would be produced by thermal excitation, In contrast with the superconducting transistor of U.S. 4,157,555, where the superconducting gap A changes by an amount much less than 1%, the superconducting gap A changes drastically in the present invention, and is generally driven to zero.
- the amount of power applied is at least of the order of the threshold power density, and generally exceeds that density, in order to change the state of the device.
- the superconducting electrodes, geometrical configuration, and type of load are chosen so that the output power of one device is sufficient to exceed the power density threshold of another identical device.
- the injector junction is generally comprised of a single tunnel barrier layer
- multiple tunnel barrier layers can be used to provide multiple inputs to the injector.
- the injector tunnel barrier can be comprised of an insulator, a normal metal, a semi-metal, or even a semiconductor.
- the barrier layer used for the acceptor tunnel junction is generally an insulator, since the acceptor junction must evidence a sharp current transition in its current-voltage curve indicating two distinct superconductor gaps.
- the acceptor tunnel junction can be comprised of a plurality of tunnel junctions in series, or can be comprised of a single tunnel junction. As will become more apparent, the use of multiple junctions in series leads to a device with higher voltage outputs, and one which offers more flexibility in fabrication.
- this superconducting device can switch from a Josephson current state (i.e., zero voltage state) to a non-zero voltage state.
- the mode of operation in this embodiment is non-inverting.
- a three-state device can be obtained in which one of the states is a Josephson current state, while the other states are non-zero voltage states.
- the acceptor tunnel barrier does not have to be comprised of an insulating material, i.e., it can be a semi- conductor, metal, or a semi-metal.
- This superconducting device is capable of high gain (greater than about 10), is latching or non-latching (depending upon the biasing), and can be fabricated to provide dense arrays. In the operating modes where Josephson currents are not used, it will not exhibit resonance properties and also has a very low sensitivity to trapped flux. It exhibits high noise immunity and provides high discrimination between output states. The amount of energy required to switch it is extremely low, being of the order of 10-" joules.
- the superconducting devices of the present invention rely on heavy injection of excess energetic quasi-particles to modify the gap A of a superconductor such that ⁇ 0, thereby greatly altering the tunneling properties of an output (acceptor) tunnel junction.
- the device is specifically designed to provide sufficient output energy (voltage) to be able to drive another such device in order to change the tunnel characteristics of the driven device. That is, when used as a switch, the device must provide a useful output, where "useful" means that the output exceeds the threshold power of all the driven Quiteron devices in orderto switch them to another state. Also, this device can provide inversion and non-latching properties, as well as high gain.
- a first tunnel junction injector junction
- a second tunnel junction i.e., output or acceptor junction
- electrical excitation optical, phonon, electron, ion, or neutron excitation, or any combination of these types of excitation can be used to inject excess energetic quasi-particles to make ⁇ 0.
- a three terminal device can use a combination of electrical excitation and optical excitation to significantly change the gap of a superconductor in the device in order to change the tunneling characteristics of the output acceptor junction.
- a superconducting strip can be driven normal (resistive) by heating it to its critical temperature T e or by passing a current through the strip whose density exceeds the critical current density Jc.
- Another technique for driving the superconducting strip normal is to apply an external field greater than the critical field He.
- the superconducting strip is driven from its superconducting state to a resistive state, the superconducting gap A disappears. This is true for both thin and thick superconducting strips, although the change'in the gap varies at a different rate depending upon the film thickness. This is the basis for operating the known cryotron switch.
- a superconducting-insulator-normal conductor structure is shown.
- a tunnel barrier 10 is located between the normal conductor N and a superconductor S.
- Barrier 10 can be, for example, an oxide of the normal metal N.
- the voltage V 12 is a voltage measured between points 1 and 2 on the superconductor S.
- This SIN tunnel junction is a means by which quasi-particles can be injected into superconductor S. This is a known structure and is used to illustrate the injection of quasi-particles.
- the tunneling current IT creates an excess quasi-particle number density of energy eV T , where N and N T are the non-equilibrium and equilibrium quasi-particle number densities, respectively, N(O) is the Bloch density of state, and A(0) is the zero temperature gap.
- the excess density n increases as the injected power density is increased until threshold values P c and n c are reached such that ⁇ (n c ) vanishes. This is illustrated in Fig. 1.2.
- the top curve 12 is derived in accordance with the known BCS(T * ) model, while the bottom curve 14 is derived from the known Owen-Scalapino ⁇ * model.
- Fig. 1.3 is a plot of the tunneling current I T as a function of the voltages V T and V 12 .
- curve 16 applies when P ⁇ P c
- curve 18 is for P>P o .
- the threshold power density Pc and its dependence on injeeted quasi-particle energy directly affect the characteristics of the Quiteron, since they determine the gain, power dissipation, and speed of this device.
- the critical injection current density J Tc can be much smaller than the critical current density J c of the superconductor S. This distinguishes the non-equilibrium mechanism of the Quiteron and the equilibrium mechanism as far as gap suppression is concerned.
- the ratio J c /J Tc is the absolute upper limit of the Quiteron gain. Another distinguishing factor is the speed of the non-equilibrium mechanism, which is much faster than that of the equilibrium mechanism.
- the quasi-particle injection is stopped (I T ⁇ 0), and the gap recovers its equilibrium value A in a time (relaxation time) governed by several mechanisms.
- These mechanisms include inelastic scattering, recombination of holes and electrons with the emission of phonons at nw>2A, phonon escape from the superconductor, and quasi-particle diffusion. These mechanisms also influence the critical power density.
- Threshold power density (Figs. 2.1 and 2.2)
- the gap A will vanish.
- the threshold power density is therefore the density which is required to observe the voltage V 12 ⁇
- the gap A remains at its equilibrium value or very near to it (i.e., the change in A is «1 %) and the voltage V 12 is not observed.
- the output voltage of that device is intended to be ⁇ 2 ⁇ and its current-voltage characteristics remain substantially the same. In turn, this means that a switching device useful for logic applications cannot be obtained, nor can an amplifier having linearity with large gain.
- the parameter d has a lower limit set by the coherence length and the proximity effect which tend to decrease the gap from its bulk value.
- the thermalization time T also depends on the thickness through the phonon escape mechanism from the film into the surroundings.
- the film S 2 is thin enough so that its gap A 2 vanishes at P c , while superconductor S 1 is much thicker and consequently is not affected by P c .
- the sharp rise of P c at ⁇ 1 is expected to occur due to the very sharply peaked densities of states at the gap edges of both materials.
- the injected quasi-particles recombine with the emission of phonons at energies greater than 2A 2 , which in turn break pairs in a time faster than they can escape from the film.
- the importance of the power in the Quiteron action is that it is a threshold quantity, i.e., when P c is reached the gap vanishes. Large currents alone, or large voltages alone, do not necessarily cause the gap to vanish. An excessive amount of current will be required if an equilibrium phenomenon is relied upon. In this situation, the critical current of the film itself must be exceeded and no pair breaking will occur until the critical current is exceeded. However, in the non-equilibrium situation caused by heavy injection of high energy quasi-particles, less power is required to have the gap A to vanish to 0 in accordance with the relation shown in Fig. 2.2. The amount of voltage which is applied is sufficient to observe pair breaking, the minimum voltage being 2 ⁇ .
- voltages are applied such that Cooper pairs break, and so that the gap vanishes.
- Relatively lower injected current and relatively lower voltage [ ⁇ ( ⁇ 1 + ⁇ 2 ) result in small power capable of making ⁇ 0.
- the energy of the injected quasi-particles and the number injected is sufficient to make a significant change in the gap ⁇ .
- Non-equilibrium phonomena are utilized, and the output voltages are sufficiently high that similar Quiteron-type devices can be driven.
- Fig. 3.1 shows a structure comprising first and second superconductors S, and S 2 , having gaps ⁇ 1 , and ⁇ 2 , respectively, and separated by a tunneling barrier 22.
- a tunnel current I flows through the tunnel barrier, and a voltage V is established across the barrier.
- the current-voltate (I-V) characteristics of the tunnel junction in Fig. 3.1 are shown in Fig. 3.2.
- the solid curve 24 is that due to an equilibrium state where P ⁇ P c . As will be more apparent later, curve 24 must be one in which a current transition occurs at ⁇ 1 + ⁇ 2 . This feature is required for the equilibrium I-V characteristic of the acceptor junction.
- the dashed curve 26 corresponds to that which occurs when P is greater than P c (i.e., ⁇ 2 ⁇ 0).
- the I-V characteristic changes from curve 24 to curve 26.
- P the threshold power density
- the tunnel device switches back to the I-V curve 24.
- the ability to switch back and forth between curves 24 and 26 is referred to as the Quiteron action. This action can be achieved by the following different external excitations:
- the arrows 28 represent the application of injected quasi-particles into superconductor S 2 , as for example by photon beams.
- the Quiteron devices of Figs. 4.1 and 4.2 illustrate embodiments wherein the injection of quasi-particles is by means of another tunnel junction, which will be called the injector junction.
- the injector could be a weak link device.
- the output junction (acceptor junction) is comprised of a single junction and the device is termed a single junction acceptor (SJA).
- SJA single junction acceptor
- Fig. 4.2 the output junction is comprised of two junctions in series and the device is called a double junction acceptor (DJA).
- DJA double junction acceptor
- the Quiteron device 30 is comprised of three superconducting films S 1 , S 2 , and S 3 , having gaps ⁇ 1 , ⁇ 2 , and ⁇ 3 , respectively.
- maximum discrimination between binary 1 and 0 states, and the ability to drive other devices occurs if certain relationships exist between ⁇ 1 , ⁇ 2 , and ⁇ 3 . Adjustment of these gaps relative to one another also affects the gain that can be obtained.
- Middle electrode S 2 is made thin enough so that its gap ⁇ 2 can be suppressed to zero when the threshold power density P c is exceeded.
- Films S 1 and S 3 are thick enough to prevent them from going normal when P c is exceeded.
- Tunnel junction 32 is termed the "injector” junction since its function is to inject quasi-particles into superconductor S 2 .
- Junction 34 is termed the "acceptor” junction since its tunneling characteristics will be modified depending upon the injection of quasi-particles into S 2 .
- S 1 can be a non-superconductor (i.e., normal) or it can go normal, without adversely affecting device operation.
- S 2 must be a superconductor whose gap can be drastically reduced, and is sufficiently thin that a reasonable input power can be used to do that.
- S 3 is preferably a thicker superconductor, since the device is faster if only S 2 has its gap drastically changed when P>P c is applied. If S 3 stays superconducting, then two distinct states are achieved and only S 2 has to return to its original ⁇ 2 state when the input power is removed. This means that switching between the different output states will be faster.
- electrode S 3 is a continuous electrode across the entire lateral extent of barrier 34.
- the device is a single junction acceptor.
- the injector and acceptor junctions are biased with DC current sources I i and Ia, and a load R L is connected across the acceptor junction formed by electrodes S 2 and S 3 having tunnel barrier 34 therebetween.
- the voltage across R L is V L .
- Fig. 4.2 shows a double junction acceptor in which two acceptor junctions are in series.
- superconductors S 1 , S 2 , and S 3 are provided having gaps ⁇ 1 , ⁇ 2 , and A3, respectively.
- S 1 and S 2 are separated by a tunnel barrier 36, while superconductors S 2 and S 3 are separated by tunnel barrier 38.
- S 2 is sufficiently thin that its gap can be suppressed to zero when P c is exceeded, while films S, and S 3 are sufficiently thick that they will not go normal when P c is applied.
- the same considerations with respect to S 1 , S 2 , and S 3 described for the SJA structure of Fig. 4.1 apply to the structure of Fig. 4.2.
- the DC bias sources I i and I a are also the same as those described with respect to Fig. 4.1, as is the linear load R L .
- the acceptor junction is comprised of two tunnel junctions in series. One of these is formed between S 3 and S 2 , while the other is formed between S 3 ' and S 2 .
- the use of at least two junctions in series for the acceptor junction leads to a Quiteron whose output voltage is >( ⁇ 1 + ⁇ 2 ) and hence it can drive another Quiteron device.
- SJA greater constraints exist on the electrode materials in order to achieve the same results.
- Fig. 4.3 shows the current-voltage characteristics (I-V) of the Quiterons of Figs. 4.1 and 4.2 across the linear resistive load R L .
- I-V current-voltage characteristics
- the acceptor tunnel barrier is generally an insulating material, such as an oxide of S 2 .
- the injector tunnel barrier can be more arbitrarily selected and can be, for instance, a normal metal, a semi-metal, an insulator, a semi-conductor, etc.
- layered structures have been shown for the SJA and DJA devices, other geometries can be used.
- the acceptor junction and/or the injector junction can be an edge junction.
- Fig. 5.1 shows two directly coupled DJA Quiterons.
- the output of the first Quiteron 44 is used to drive the input of the second Quiteron 46.
- the injector junction of Quiteron 46 represents a nonlinear load to the output of Quiteron 44. As will be more apparent with respect to Fig. 5.2, this provides a nonlinear load line leading to a circuit having very good discrimination between different output states.
- Quiteron 44 is comprised of electrodes S l , S 2 , and S 3 , having superconducting gaps ⁇ 1 , ⁇ 2 , and A3, respectively.
- Tunnel barrier 48 is provided between S, and S 2
- tunnel barrier 50 is provided between S 2 and S 3 , as well as between S 2 and S 3 '.
- S 3 and S 3 ' are comprised of the same material.
- Quiteron 46 is also comprised of three electrodes having the same designations S l , S 2 , and S 3 .
- the superconducting gaps are ⁇ 1 ⁇ 2 , and ⁇ 3 .
- Superconductor S 3 ' is typically the same as S 3 , and therefore also has the gap ⁇ 3 .
- Tunnel barriers 52 and 54 are provided in the same manner as tunnel barriers 48 and 50.
- the materials comprising electrodes of Quiteron 46 will be the same as the material providing the electrodes in Quiteron 44. Additionally, the tunnel barriers 48 and 50 will be comprised of the same material as tunnel barriers 52 and 54, and will be formed at the same time.
- a current I i is provided to the injector of Quiteron 44, and is designated as input A. Since the Quiteron is an inverter, the output of Quiteron 44 is A.
- the acceptor junction 50 of Quiteron 44 is biased with DC current I a1 while the acceptor junction 54 of Quiteron 46 is biased with DC current I a2 .
- a load 56 is provided across Quiteron 46.
- Quiteron 44 It is possible for Quiteron 44 to drive Quiteron 46 since the gain of Quiteron 44 is sufficiently high that its output power in the "1" state, P 1 , exceeds the threshold power of Quiteron 46, while that of the "0" state, P o , does not.
- Quiteron 46 represents a nonlinear load to Quiteron 44 and this results in a significant increase in the discrimination ratio D ⁇ P 1 /P 0 between the "1" and "0" states. In order to take full advantage of this nonlinear loading, the condition is satisfied.
- R j , V m , R NN are defined in Fig. 6, and I o ⁇ 0.7( ⁇ 2 + ⁇ 3 )/R NN .
- R NN is the normal tunnel resistance
- V m describes the quality of the junctions which ranges between 25 mV and 50 mV for lead alloy junctions.
- Fig. 5.2 shows the I-V characteristics of the acceptor junction 50 with the nonlinear load represented by Quiteron 46.
- the solid curve 58 is the curve followed when the input excitation to Quiteron 44 is less than the threshold power density, while the dashed curve 60 is the I-V characteristic when the threshold power density of Quiteron 44 is exceeded such that ⁇ 2 vanishes.
- the load line has two portions: A first portion 62 representing a high resistance and a second portion 64 representing a lower resistance. The binary "1" state occurs where the load line intersects solid curve 58, while the "0" state occurs where the load line intersects dashed curve 60.
- Fig. 6 is a I-V plot of the acceptor characteristics of Quiteron 44 when the load for this Quiteron is a nonlinear load, and in particular is Quiteron 46.
- Fig. 6 is essentially the same as Fig. 5.2, except that it is used to define the parameters which are used to derive the expressions for the discrimination ratio (Equation 2) and for the gain of the device (Equation 7).
- Equation 2 the discrimination ratio
- Equation 7 the gain of the device
- 2A3 is less than ⁇ 1 + ⁇ 2 , which in turn is less than 2(A 2 +A 3 ).
- Fig. 7 is a plot of the threshold power density P c as a function of the normalized quasi particle energy
- Fig. 7 shows the location of the binary states with respect to the threshold power density curve, and is used to indicate the enhancement of discrimination when a nonlinear load is applied to Quiteron 44.
- Equation 2 for the discrimination factor D has to be multiplied by the ratio P c (0)/P c ( ⁇ ) to obtain
- the Quiteron devices do not require the . presence of Josephson tunneling current through the tunnel barriers. Therefore, resonance problems do not arise and these devices are not ultrasensitive to trapped flux.
- Josephson current can be present, as will be explained with respect to the embodiments of Figs. 21.1, 21.2, 22, and 23.
- the power gain G of the Quiteron is the ratio of the output power across the load to the injected threshold power P c A i where A i is the injector junction area. This gain is given by the following expression: Let I a ⁇ 1/2j a A a , and and assuming that j i is approximately 1/2j Tc and the gain can be rewritten as
- Equation 6 indicates that by increasing the current density ratio it is possible to obtain G»1. It also indicates that the gain does not depend on the lateral dimensions of the Quiteron, but instead depends upon the tunnel barrier thicknesses.
- the Quiteron device is potentially miniaturizable.
- the tunnel barrier thicknesses of the acceptor and injector barriers differ by merely 3 angstroms.
- the ratio J a /J i is not arbitrarily chosen; instead, j i is chosen such that Gain is then defined after the threshold is reached.
- the ratio j a /j i in the transistor of U.S 4,157,555 is arbitrarily chosen.
- G max is given by the following expression:
- Equation 6 Since the gain as given by Equation 6 does not depend upon the lateral dimensions of the Quiteron, advanced lithographic techniques and processes can be used to miniaturize these devices. At small line widths, the gain will be affected by the quasi-particle diffusion length. Since the operation of this device is based on overinjection of quasi-particles, the diffusion length decreases to about 100 nm, which is assumed to fix the smallest line width.
- the non-equilibrium phenomena used to change the state of this device provides quite high switching speeds.
- superconductors having high T c , and their alloys should have the shortest quasi-particle relaxation times.
- the most dominant relaxation processes are recombination to form a bound Cooper pair with the emission of a phonon, and inelastic scattering with phonons.
- the recombination times and inelastic times become shorter at higher energies, which further proves the significance of the double junction acceptor embodiment.
- the three electrode devices shown herein use a very thin superconductor S 2 whose gap is modified by overinjection of quasi-particles. Electrical contact must be made to this very thin electrode. Additionally, the electrodes S 1 and S 3 are thicker, and this means that the thin middle electrode S 2 has to step down a thick electrode S l .
- Fig. 8 shows a scheme for making electrical contact to the thin electrode S 2 .
- Fig. 8 shows the electrodes S i , S 2 , and S 3 , which are separated by the injector barrier 66 and acceptor barrier 68.
- the injector. junction 66 is thinner in the region where the Quiteron action occurs, but is thicker in the region where electrical contact is to be made to S 2 .
- the thick metallurgical layer 70 is used to contact S 2 .
- acceptor barrier 68 can be formed from an insulator which is thicker in the region outside of electrodes S 3 in the same manner that the injector barrier is thicker outside the device area. This will further ensure that there is no undesired electrical contact between conductor 70 and S 3 .
- Fig. 9 shows a top view of a layout for a double junction acceptor device such as that in Fig. 8.
- the bottom electrode is S i
- the thin middle electrode is S 2 .
- the top electrode is S 3 .
- the additional thick metallurgy layer 70 is used to make contact to thin electrode S 2 and is shown in the side view of Fig. 8.
- the acceptor junction 68 is a duoble junction, while the injector junction 68 is formed between S 1 and S 2 .
- the injector current flows into contact strip 70 and out of S 1 .
- Layer 72 is an insulating layer which electrically insulates S 1 from S 3 . It is, for example, a layer of SiO.
- W is the minimum linewidth.
- this device layout occupies an area 21 W 2 .
- One disadvantage of the Quiteron is in the required control of the current densities j i and j a to within reasonable tolerances. However, this disadvantage may be rendered less severe due to the availability of gain, large discrimination factor, and high noise immunity.
- This device has applications in digital: systems, analog systems, and in the study of the physics of non-equilibrium superconductivity.
- the technology necessary to make superconductive tunneling devices, such as Josephson tunneling devices, can be used to fabricate circuits of Quiterons. The following sections will describe the use of these devices in logic circuits, and as analog amplifiers.
- Quiteron logic circuits (Figs. 10.1, 10.2, 11.1, 11.2, 12-18) 1
- NOR logic circuits can be built using these devices. This provides advantages in fabrica- tion/density/scaling over the Josephson device circuits, in which AND, OR, and INVERT blocks are needed to design complete logic circuits.
- the Quiteron is an inverting device, a NOR block can easily be designed, and this can be used to build a complete family of logic circuits.
- the SJA configuratiion . is more . suitable than the DJA configuration.
- the DJA configuration has an advantage from the fabrication and metallurgical points of view. These configurations can be combined in a single circuit, as will be apparent from the full adder of the Fig. 17.
- Fig. 10.1 shows an SJA configuration
- Fig. 10.2 shows the I-V characteristic.
- the ratio of the 1 voltage state to the 0 voltage state is These voltages are indicated in Fig. 10.2 for the SJA device having an injector current I i and an acceptor bias current I a .
- the injector junction 74 is located between S, and S 2 while the acceptor junction 76 is located between S 2 and S 3 .
- a nonlinear load 78 is connected across the acceptor junction and can be, for example, another SJA device.
- V(1)/V(0) is approximately ( ⁇ 2 / ⁇ 3 )+1 ⁇ 1.5
- ⁇ 2 / ⁇ 3 is at least equal to or greater than 0.5.
- Fig. 11.1 shows a DJA configuration
- Fig. 11.2 gives the I-V characteristics and indicates the voltages when the device is in its 0 and 1 states.
- the injector junction 80 is located between S 1 and S 2
- the acceptor junction 82 is located between S 2 and S 3 .
- a nonlinear load 84 preferably another DJA configuration, is connected across the acceptor. I i provides DC bias to the injector junction, while I a provides DC bias to the acceptor junction.
- V(1)/V(0) is 2( ⁇ 2 + ⁇ 3 )/( ⁇ 1 + ⁇ 2 ).
- Fig. 12 shows DJA devices powered by current sources I a1 , I a2 , and l a3 in order to obtain a fan-out of four.
- the DJA Quiterons are shown schematically, and are labelled Q 1 , Q 2 , Q 3 , Q 4 , and Q 5 .
- the loads L 2 , L 3 , L 4 , and L 5 are connected across devices Q 2 ⁇ Q 5 , respectively.
- Fig. 13 illustrates a parallel powering scheme which requires a single voltage bus V S and a power resistor R s for each device. This scheme dissipates more power than a series connected DJA configuration, but takes better advantage of Quiteron density.
- the power resistor R s can be made with a suitable tunnel junction, especially since the supply voltage V S is a DC voltage.
- device Q 1 has an injector input A, and will provide an acceptor output A when A exceeds the threshold power density. Only when input A is present to device Q 2 will the outputs A be realized from Q 2 .
- A is also an input to device Q 3 . Its output will be A, which is provided to the acceptor electrode of device Q 4 . Only when input B is present to Q 4 , will the output 8 be provided from Q 4 . Thus, the presence of A to Q 3 at the same time that B is provided to Q4 is required to produce the output A B.
- Fig. 14.1 shows a three input NOR circuit with Quiterons Q,, Q 2 , and Q 3 .
- a single voltage bus delivers the power V S through a resistor R s .
- acceptor current I a flows to each of the acceptors of devices Q 1 ⁇ Q 3 .
- Input A is provided to the injector of Q 1
- input B is provided to the injector of Q 2
- input C is provided to the injector of Q 3 .
- the presence of any input A, B, or C will provide an output A or B or C, respectively.
- the I-V characteristics of the three input NOR circuit of Fig. 14.1 are shown in Fig. 14.2.
- devices Q 1 ⁇ Q 3 assume the equilibrium characteristics given by the solid curve 86, and nearly all of the current I a is delivered to the load (not shown).
- This load is the injector nonlinear characteristics of the fan-out Quiterons.
- the non-equilibrium characteristics represented by the dashed curves 88, 90, and 92 are developed, causing the load voltage to switch to A3 and the current to diminish to ⁇ 3 /R j (load).
- Curve 88 appears when one of the inputs is present while curve 90 appears if two inputs are present.
- Dashed curve 92 appears when all three inputs are present.
- Fig. 15.1 shows a multi-injector NOR configuration which is nearly identical to the three input NOR circuit made of discrete Quiterons, as shown in Fig. 14.1.
- a single acceptor is provided, while three separate injectors are provided, having inputs A, B, and C.
- a voltage bias V S is provided through resistor R s to the acceptor junction.
- the output A or B or C will be provided.
- Fig. 15.2 The I-V characteristics of Fig. 15.2 are otherwise the same as those of Fig. 14.2.
- solid curve - 94 is the curve followed when no input is present
- dashed curve 96 is the curve followed when one input is present
- dashed curve 98 is the one followed when two inputs are present
- dashed curve 100 is the curve followed when all three inputs are present.
- Fig. 16 shows a four-bit decoder using parallel powered NOR blocks.
- the circuit is comprised of the single injector SJA devices Q 1 , Q 2 , Q 3 , Q 4 , etc.
- Multi-injector SJA devices Q 5 , Q 6 , Q 7 ,...,Q 20 are also provided.
- the acceptor outputs of Q 5 ⁇ Q 20 provide the decoder outputs.
- the decoder structure of Fig. 16 is similar to other decoders known in the art. Depending upon the injector inputs A 1 , A 2 ; A3, and A4, devices Q 1 ⁇ Q 4 will provide outputs A 1 ⁇ A 4 . The outputs of Q,-Q 4 , as well as the inputs A 1 ⁇ A 4 , are provided in different combinations to the multiple injectors of Q 5 ⁇ Q 20 . As is well known in the decoder art, depending upon the combinations of the four inputs, 16 outputs 0 1 -0 16 can be obtained.
- Fig. 17 shows a one-bit full adder using both SJA and DJA device configurations.
- the design of this circuit is similar to the design of other full adders and provides as an output the complement S n of the sum signal and the complement C n of the carry signal.
- the adder is comprised of three SJA devices Q 1 ⁇ Q 3 at the circuit input, and three double injector Quiterons Q 5 ⁇ Q 7 . Additionally, five triple-injector Quiterons Q 8 ⁇ Q 12 are used as well as one quadruple injector Quiteron Q 13 .
- the acceptor junctions of all the Quiterons are provided with the same supply voltage V S through resistors R s .
- Devices Q 1 ⁇ Q 3 provide the complements of the inputs X n , Y n , and C n-1
- the outputs from the acceptors of Q 1 ⁇ Q 3 are applied to the injector junctions of Quiterons Q 9 ⁇ Q 11 .
- Devices Q 5 ⁇ Q 8 receive as injector inputs various combinations of the circuit inputs X n , Y n , and C n-1
- the acceptor outputs of devices Q 8 ⁇ Q 11 are applied as injector inputs to Q 13 to develop the complement S n
- the acceptor outputs of devices Q 5 ⁇ Q 7 are the injector inputs to Q 12 , to provide the complement C n of the carry signal.
- a Quiteron is an inverter device, it can be utilized to design a flip-flop circuit, such as the NOR flip-flop shown in Fig. 18.
- a circuit of this type can be used to provide memory latches, as well as memory arrays when an appropriate selection scheme is developed.
- the flip-flop is comprised of two double injector Quiterons Q 1 and Q 2 which are cross-coupled to one another.
- a DC bias V s is applied to the acceptor junction of each device Q 1 and Q 2 through resistances R s .
- a SET signal S is applied to one injector input of Q i , while a RESET signal R is applied to one injector input of Q 2 .
- the other injector input of Q 1 is connected, via line 102, to the acceptor output of Q 2 , while the second injector input to Q 2 is connected to the acceptor output of Q 1 via electrical connection 104.
- the operation of the flip-flop is the same as conventional flip-flops, a true signal T and a complement signal C being provided at the acceptor outputs of Q 1 and Q 2 , respectively.
- the Quiteron structure can be used to provide analog amplification of the input signals to its injector junction. For this use, it is desirable to change the superconducting gap drastically, but it is not necessary to reduce the gap to zero. In fact, if the gap is reduced to zero, a linear output will not be obtained. Since linearity is important in an amplifier, the input signal should be less than that which would cause the gap of the superconductor into which quasi-particles are injected from vanishing.
- Fig. 19 shows the current-voltage characteristics of a Quiteron device used as an analog amplifier.
- the dashed curve 108 is the curve which would result if ⁇ 3 + ⁇ 2 ⁇ 0.
- a threshold type of operation is necessary for digital switch applications, but this threshold may not be reached when amplification is desired.
- a family of curves 110, 112, 114, 116, 118, 120, and 122 can be developed. As ⁇ 2 diminishes, the I-V curves change from 106 and 110, then to 112, etc.
- the amplifier can be biased at a voltage V 1 , which corresponds to a power input P 1/2 P c , and the presence of an input signal causes an output which is determined by the linear voltage of the load line 124.
- V 1 a voltage which corresponds to a power input P 1/2 P c
- output voltages from V 2 to V 3 can be obtained depending upon the input signal magnitude.
- Large current amplification can be obtained in this way, in contrast with the amplifier of U.S. 4,157,555 in which P «P c . It is not possible to obtain large signal gain in that prior art transistor. In contrast with this, the superconducting gap is drastically changed to provide a full family of curves 110-122 in order to provide a large gain.
- injection of quasi-particles from a tunnel barrier provides over injection of quasi-particles into the superconductor whose gap is to change.
- This type of injection input can be modified to use a combination of inputs, such as electrical inputs and photon beams.
- a Quiteron can be fabricated in which an electrical signal is applied to the injector junction as well as a photon beam.
- the input excitation can be due to only the photon beam in order to provide a full optical transistor.
- the thicknesses of the various electrode layers are chosen with respect to the device characteristics that are required.
- the thin superconducting electrode S 2 whose superconducting gap must be drastically altered should be sufficiently thin that this can be achieved without the application of unduly large signals.
- a representative thickness for the electrode S 2 is 50-100 nm. This is dictated by the fact that the threshold power density P c must be reached of reasonable power levels.
- the acceptor current density can now be chosen to obtain the desired gain which is given by j a /j i ⁇ 100, for example.
- the thicknesses of the tunnel barriers are chosen ⁇ to provide appropriate ratios of the current densities j. and j,. While the presence of Josephson tunneling current through either of the junctions is not required for operation of this device, tunnel barriers can be provided where Josephson current is present. This current could be used to provide a device with three states, which is capable of latching. Representative values for the thicknesses of the injector and acceptor tunnel junctions are such that a current density ratio j a /j i ⁇ 100 is obtained. Devices employing Josephson currents, and devices using different biasing will now be described.
- a SJA device is shown in Fig. 20.1, while its I-V characteristic is shown in Fig. 20.2.
- Quiteron 126 is comprised of three electrodes S 1' S 2 , and S 3 , the injector junction 128, and the acceptor junction 130.
- Current source 132 provides injection current l i while the acceptor junction 130 is biased by current source 134, which produces current l a .
- load 136 is connected across S 1 and S 3 in this embodiment.
- the equilibrium curve 138 has a current transition at ⁇ 1 +2 ⁇ 2 + ⁇ 3 , in contrast with the usual equilibrium curve for a SJA device, which has a current transition at ⁇ 2 + ⁇ 3 .
- a greater diacrimination can be obtained between the "1" and "0" states.
- the total equilibrium voltage curve depends upon the voltages across both the injector and acceptor barriers. For this reason, a larger voltage swing can be obtained than with the other SJA embodiments.
- the voltage swing is not necessarily greater than that which is obtained with a DJA structure, and the biasing technique of Fig. 20.1, is not as satisfactory from hhe standpoint of isolation between the output and the input.
- Fig. 21.1 shows a SJA structure including electrodes S 1 , S 2 , and S 3 .
- S 2 and S 3 are superconductors, while S 1 can be a superconductor or a normal material.
- Injector tunnel barrier 140 is located between S 1 and S 2
- acceptor tunnel barrier 142 is located between S 2 and S 3 .
- a linear load 144 is connected to the acceptor electrode S 3 .
- Injector current l i flows into S 1 and out of S 2 , while the acceptor is biased by current l a which flows into S 3 and out of S 2 . If barrier 142 is sufficiently thin, a zero-voltage current will tunnel through this barrier in accordance with the Josephson effect.
- the acceptor can be comprised of a weak link if one of the device output states is a zero-voltage current.
- tunnel junction is meant to include a weak link device as well as one having electrodes separated by a tunnel barrier, i.e., any device exhibiting a Josephson current.
- ⁇ is a single flux quantum
- l o is the Josephson critical current through tunnel barrier 142
- R is the total junction resistance
- C i is the junction capacitance.
- Curve 146 is the equilibrium curve of the Quiteron where ⁇ «1 (i.e., a hysteretic junction) while curve 150 represents the equilibrium state when ⁇ «1.
- P>P the characteristic switches to that depicted by curve 148 or curve 152, depending on the amount of power overdrive.
- FIG. 21.2 two load lines having slopes Rl 1 and R L2 are shown.
- a zero-voltage Josephson state can occur when Josephson current tunnels through the acceptor junction. This occurs for ( ⁇ «1.
- P increases to P c , the device switches from a Josephson state to a state given by the intersection of the load line and curve 152.
- this Quiteron is non-inverting, and non-latching.
- Fig. 22 shows the I-V characteristics of a Quiteron exhibiting a 0-voltage Josephson state, where the load is now a nonlinear load, such as that represented by another Quiteron device.
- Equilibrium curve 154 represents the I-V characteristic for a junction having ⁇ »1 (hysteretic), while the solid line curve 156 represents the equilibrium curve for a junction in which ( ⁇ «1 (non-hysteretic).
- Curves 154 and 156 are the equilibrium curves when P «P c .
- Nonlinear load line 162 intersects curve 156 at a point corresponding to a binary "0", and intersects curve 160 at a point representing a binary "1" state.
- the device of Fig. 22 is non-inverting, and non-latching.
- Fig. 23 shows an I-V curve for the Quiteron of Fig. 21.1 biased to have three states, where one state is a Josephson zero voltage state.
- the Quiteron of Fig. 21.1 has an equilibrium curve 164 and, if the acceptor barrier is sufficiently thin and there is magnetic shielding, a Josephson current can flow through the acceptor barrier.
- ( ⁇ »1, and the load is linear, as shown by linear load line 166.
- Curve 168 represents the situation when P>P c .
Landscapes
- Superconductor Devices And Manufacturing Methods Thereof (AREA)
Description
- This invention relates to a three terminal superconducting tunnel junction device comprising three conducting electrodes which are separated from one another by two tunnel barriers to form two tunnel junctions. More particular, the invention concerns a superconducting device whose current-voltage characteristic is altered by heavy injection of excess quasi-particles. The device can be used for logic operations, or as an amplifier, and has an output that is sufficiently large to drive another such device.
- Superconducting switching devices are well known in the art, and are represented by devices such as the cryotron and the Josephson tunneling device. In the cryotron, the superconducting gap of a film is caused to vanish (i.e., the film becomes normal). This can occur by several mechanisms, including passing a current through the film having a magnitude greater than the critical current of the film applying an external magnetic field, and by heating the film.
- A Josephson tunneling device is one which exhibits a zero-voltage current between two superconductors separated by a very thin tunnel barrier. By increasing the current through the device, or by applying a magnetic field to the device, the device can be made to switch to a non-zero voltage state. However, the device remains superconducting at all times.
- Another superconducting device is that represented by the transistor of U.S. 4,157,555. In that device three superconductors are separated from one another by two insulating layers to form two tunnel junctions. Depending on the biasing applied to these tunnel junctions, a. change can result in the total current flow through one of the junctions, which change is greater than the amount of current injected into one of the superconductors. In this device, quasi-particles are injected into the middle electrode, but the number of quasi-particles injected is small. That is, the number is in the order of the number of thermally axcited quasi-particles in the superconductor. This'means that the superconducting gap A of the superconductor into which the quasi-particles are injected remains near its equilibrium value. In that device, any change in A due to the. quasi-particle injection is much less than 1% of its value at equilibrium.
- The device of U.S. 4,157,555 cannot function as a switch in the digital sense. Because the superconducting gap remains near its equilibrium value, that device functions only as a small signal analog amplifier (even if the tunnel current density of its output junction is much greater than the tunnel current density of its input junction) and it is not possible to provide a sufficiently large output to drive another similar device. Stated another way, the output of that device is not sufficiently large to change the state of another such device being driven by it.
- In the device of U.S. 4,157,555, the difference in tunnel resistance between the two tunnel junctions of the device is such that the device is restricted to small signal analog amplification. In that device, recombination times must be very long with respect to the tunneling rate, and for this reason aluminum is chosen as a representative superconductor. However, these materials are not favorable if large signal amplification is desired, or if it is desired to have a switch which has distinguishable output states and which can drive another such switch.
- The transistor of U.S. 4,157,555 is not an inverter and cannot operate as a switch in a digital sense. Its collector junction is biased at an amount less than the gap voltage and for this reason, as well as the small number of quasi-particles injected, a small output results. At the present time, there is no known cryogenic switch which provides digital inversion, as can be obtained with semiconductor bipolar and FET devices.
- The operation of the present device is based on QUasi-particle /njection Tunneling, and hence is called QUITERON. It relies on heavy injection of excess quasi-particles in order to cause a superconducting gap to drastically change, and generally to vanish. It distinguishes from the experiments of Testardi, Phys, Rev. 84, 2189 (1971) in that a complete three terminal device is obtained capable of digital switching. In Testardi's experiments, he discovered that thin superconducting films can be irradiated by optical pulses to drive them into a normal state and back to the superconducting state very rapidly. Testardi suggested that an excess number of quasi-particles larger than the usual thermal equilibrium number were responsible for this phenomenon. However, no devices were suggested or described.
- Non-equilibrium superconductivitVis-â subject of intense investigation, as represented by the work of Ginzberg, Phys. Rev. Lett., 8, 204 (1962). In that reference, Ginzberg describes a double junction configuration used to measure quasi-particle recombination life-time. Still further, K. E. Gray, Appl. Phys. Lett., 32, 392 (1978) reported measurement of small signal gain in a double junction configuration and found that, at an extremely low level of injection, its operation was analogous to that of a transistor. This operation is described more fully in the Gray patent previously referenced (U.S. 4,157,555).
- The invention as claimed is intended to remedy the drawbacks hitherto encountered. In contrast with the prior art, a cryogenic switch usable for digital switching and large signal analog amplification is described. This switch uses heavy injection of excess energetic quasi-particles into a thin superconducting film, in order to drastically change the superconducting gap of this film. In most applications, the superconducting gap is driven to zero. This device can be used to switch other devices since it provides a large output, and can be used to build a NOR block. This NOR block can in turn be used to design all types of logic circuits, without the requirement for separate AND, OR, and INVERT circuits, as would be needed when using other types of cryogenic switches.
- The present device is based upon the discovery of an unexpected threshold power density relationship in which the current-voltage characteristics of the device drastically change when the superconducting gap A changes by a large amount. This threshold power density relationship leads to a switch exhibiting high gain, inverting characteristics, high noise immunity, and high discrimination between its output states.
- The superconducting device is comprised of a plurality of conducting layers which function as electrodes, there being tunnel barriers between the electrodes. A tunnel junction, termed an injector junction, is used to heavily inject excess quasi particles into a superconducting layer which is one electrode of a second tunnel junction, termed the acceptor junction. Due to the heavy injection of excess quasi particles, the current-voltage characteristics of the acceptor tunnel junction are changed. In this manner different voltage states are provided which are easily distinguishable from one another. This device can provide an inverting characteristic, and is the only known cryogenic device which has this property. Because of this, it can be used to design circuits, such as NOR blocks, which are similar to those designed with bipolar and FET devices.
- Since the device is capable of a multiple number of output states which are clearly distinguishable from one another, it can be used as a switch. Due to the large output that results, one such switch can be used to drive another in order to make complete circuits comprised of these switches. An additional application is that of an analog amplifier in which analog signals can be greatly amplified. For this use, heavy injection of excess quasi particles is also required.
- The modification of the tunneling properties of the acceptor junction depends on a heavy injection of an excess number of quasi particles. By "heavy", it is meant that the superconducting gap A of the superconducting layer comprising an electrode of the acceptor junction is drastically changed, and is usually driven to A=0. The number of quasi-particles injected is greater than the usual thermal equilibrium number of quasi particles, and is typically at least two orders of magnitude greater than the number which would be produced by thermal excitation, In contrast with the superconducting transistor of U.S. 4,157,555, where the superconducting gap A changes by an amount much less than 1%, the superconducting gap A changes drastically in the present invention, and is generally driven to zero.
- In the practice of this invention, it has been discovered that a threshold power density exists at which an excess quasi particle concentration causes the order parameter (superconducting gap) to locally vanish. Thus, when the present invention is used as a switch, the amount of power applied is at least of the order of the threshold power density, and generally exceeds that density, in order to change the state of the device. The superconducting electrodes, geometrical configuration, and type of load are chosen so that the output power of one device is sufficient to exceed the power density threshold of another identical device.
- While the injector junction is generally comprised of a single tunnel barrier layer, multiple tunnel barrier layers can be used to provide multiple inputs to the injector. The injector tunnel barrier can be comprised of an insulator, a normal metal, a semi-metal, or even a semiconductor. On the other hand, the barrier layer used for the acceptor tunnel junction is generally an insulator, since the acceptor junction must evidence a sharp current transition in its current-voltage curve indicating two distinct superconductor gaps. The acceptor tunnel junction can be comprised of a plurality of tunnel junctions in series, or can be comprised of a single tunnel junction. As will become more apparent, the use of multiple junctions in series leads to a device with higher voltage outputs, and one which offers more flexibility in fabrication.
- In still other embodiments, this superconducting device can switch from a Josephson current state (i.e., zero voltage state) to a non-zero voltage state. The mode of operation in this embodiment is non-inverting. In another mode of operation, a three-state device can be obtained in which one of the states is a Josephson current state, while the other states are non-zero voltage states. In this latter devices, the acceptor tunnel barrier does not have to be comprised of an insulating material, i.e., it can be a semi- conductor, metal, or a semi-metal.
- This superconducting device is capable of high gain (greater than about 10), is latching or non-latching (depending upon the biasing), and can be fabricated to provide dense arrays. In the operating modes where Josephson currents are not used, it will not exhibit resonance properties and also has a very low sensitivity to trapped flux. It exhibits high noise immunity and provides high discrimination between output states. The amount of energy required to switch it is extremely low, being of the order of 10-" joules.
- Different ways of carrying out the invention are described in detial below with reference to drawings which illustrate specific embodiments, in which:
- Fig. 1.1 shows a known superconductor-insulator-normal metal structure for injection of quasi-particles into the superconductor.
- Fig. 1.2 is a curve illustrating the dependence of the supercoducting gap on the excess quasi-particle density at T=0°. These curves are based on two models relating the superconducting gap to the excess quasi-particle density 9.
- Fig. 1.3 is a plot of the current IT through the superconductor-insulator-normal metal structure of Fig. 1.1 plotted as a function of the voltage VT across the structure of Fig. 1.1 and the voltage V12 measured between two points in the superconducting layer.
- Fig. 2.1 shows a structure comprising two superconductors S1 and S2 separated by a thin tunneling barrier, while Fig. 2.2 shows the expected dependence of threshold power density Pe(s) on injected quasi-particle energy s.
- Fig. 3.1 is an illustration of a structure including two superconductors S1 and S2 where quasi-particles are injected into S2, while Fig. 3.2 shows the modification of the current-voltage (I-V) characteristics of the structure of Fig. 3.1 by heavy injection of quasi-particles such that Δ2→0.
- Fig. 4.1 illustrates a three terminal superconducting switch in accordance with the present invention in which a single junction acceptor (SJA) has its tunneling characteristics modified by excess injection of quasi-particles, there being a linear load RL connected across the acceptor junction.
- Fig. 4.2 illustrates a superconducting switch in accordance with the present invention wherein two acceptor junctions in series have their tunnel characteristics modified by injection of excess quasi-particles into the acceptor junctions, there being a linear load RL connected across the series connection of acceptor junctions.
- Fig. 4.3 shows the current-voltage (I-V) of the SJA and DJA structures of Figs. 4.1 and 4.2, and illustrates the "1" and "0" states produced across the resistive load RL.
- Fig. 5.1 shows two directly coupled Quiterons, where a first double junction 'acceptor has as a load a second double junction acceptor. The second DJA provides a nonlinear load to the first DJA, and the current-voltage characteristics of the first DJA are shown in Fig. 5.2.
- Fig. 6 is another plot of the acceptor characteristics of a first DJA having a second DJA as a non-linear load, and is used to provide the definitions used for the derivation of gain and discrimination ratio; where one DJA Quiteron drives another DJA Quiteron, as shown in Fig. 5.1.
- Fig. 7 is another plot of power density Peas a function of injected quasi-particle energy s, showing the location of the binary states of the Quiteron, thereby indicating the enhancement of discrimination between the "1" and "0" states.
- Fig. 8 illustrates a scheme for making electrical contact to the thin superconducting electrode S2 whose gap is greatly modified by the introduction of excess quasi-particles. A DJA structure is shown in this side view.
- Fig. 9 is a top view of the DJA structure of Fig. 8, showing a possible layout of this Quiteron device, where the superconducting layers are Si, S2 and S3.
- Fig. 10.1 shows a SJA Quiteron which can be used in a logic circuit, while Fig. 10.2 shows its current-voltage characteristic.
- Fig. 11.1 shows a DJA Quiteron which can be used in a logic circuit, while Fig. 11.2 shows its current-voltage characteristics for this application.
- Fig. 12 shows a DJA Quiteron circuit powered by current sources la1, la2' and 1a3, in order to obtain a fan-out of four.
- Fig. 13 illustrates SJA Quiteron parallel powered logic circuits. This SJA Quiteron configuration avoids isolation problems encountered by DJA Quiteron circuits such as those in Fig. 12.
- Fig. 14.1 shows a three-input NOR circuit using SJA devices, while Fig. 14.2 shows the curernt- voltage characteristic of this NOR circuit.
- Fig. 15.1 shows a three-input NOR circuit using a SJA device which has a multi-injection input (i.e., the injector junction is comprised of a plurality of tunnel junctions).
- Fig. 15.2 shows the current-voltage characteristic of the device of Fig. 15.1.
- Fig. 16 shows a four-bit decoder using SJA devices, some (Q1-Q4) of which have single injector junctions, while others (Q5-Q20) have multi-injector junctions.
- Fig. 17 shows a full adder using Quiteron NOR circuits, and which includes both single junction and multiple injector junction devices.
- Fig. 18 shows a Quiteron flip-flop circuit, using double injector SJA Quiterons.
- Fig. 19 shows the current-voltage characteristics of a DJA Quiteron used as an amplifier for amplifying analog signals.
- Fig. 20.1 shows a SJA device using a different biasing scheme, while Fig. 20.2 shows the I-V characteristic of the device of Fig. 20.1.
- Fig. 21.1 shows a Quiteron device with a linear load, while Fig. 21;2 shows its I-V characteristics, depending on different values of Stewart-McCumber hysteresis parameter (3, and on different values of injected input power P, when the device switches from a Josephson state to another state.
- Fig. 22 shows the I-V characteristics of the device of Fig. 21.1 when it is switched from a Josephson state to another state, where the load is now a non-linear load (such as another Quiteron device).
- Fig. 23 shows the I-V characteristics of the deyice of Fig. 21.1 when it is biased to have three distinguishable states, one of which is a Josephson state.
- The superconducting devices of the present invention rely on heavy injection of excess energetic quasi-particles to modify the gap A of a superconductor such that △→0, thereby greatly altering the tunneling properties of an output (acceptor) tunnel junction. The device is specifically designed to provide sufficient output energy (voltage) to be able to drive another such device in order to change the tunnel characteristics of the driven device. That is, when used as a switch, the device must provide a useful output, where "useful" means that the output exceeds the threshold power of all the driven Quiteron devices in orderto switch them to another state. Also, this device can provide inversion and non-latching properties, as well as high gain.
- In a general sense, the provision of three terminal devices is desirable from a circuit design standpoint, and the present invention provides such devices. A first tunnel junction (injector junction) is provided for heavy injection of excess energetic quasi-particles into a superconductor which is one electrode of a second tunnel junction (i.e., output or acceptor junction). In addition to electrical excitation, optical, phonon, electron, ion, or neutron excitation, or any combination of these types of excitation can be used to inject excess energetic quasi-particles to make △→0. As an example, a three terminal device can use a combination of electrical excitation and optical excitation to significantly change the gap of a superconductor in the device in order to change the tunneling characteristics of the output acceptor junction.
- Thus, it is an important feature of the present invention to provide a superconducting device having a useful power output and for this reason the excitation is chosen in order to create a non-equilibrium state in the superconductor whose band gap is greatly modified by the heavy injection of quasi-particles. An excitation mechanism which leaves the superconducting gap near its equilibrium value is not useful when providing switching action in the digital sense (i.e., when providing a sufficient output to drive other similar devices).
- A superconducting strip can be driven normal (resistive) by heating it to its critical temperature Te or by passing a current through the strip whose density exceeds the critical current density Jc. Another technique for driving the superconducting strip normal is to apply an external field greater than the critical field He. When the superconducting strip is driven from its superconducting state to a resistive state, the superconducting gap A disappears. This is true for both thin and thick superconducting strips, although the change'in the gap varies at a different rate depending upon the film thickness. This is the basis for operating the known cryotron switch.
- These mechanisms for driving a superconducting strip normal are equilibrium phenomena because the Cooper pairs, quasi-particles, and the phonoms involved are in thermal and chemical equilibrium with each other. For equilibrium situations, the energy distributions for the quasi-particles and phonons are given by the Fermi-Dirac and Bose-Einstein statistics, respectively. These equilibrium phenomena are well known in the field of superconductivity, and are explained more completely in the text by M. Tinkham "Introduction to Superconductivity" published by McGraw-Hill, 1975. The excess quasi-particles are created when energies greater than 2Δ are applied so that free electrons and holes are created when the Cooper pairs are split.
- It is possible to drive the superconducting strip far from equilibrium by means of overinjection of quasi-particles having energies greater than the gap Δ. The energy gap vanishes as the quasi-particles and phonons assume distributions significantly different from those associated with their thermal equilibrium values. This overinjection of excess quasi-particles causes △ to vanish. It is important to note that in this non-equilibrium excitation the films are still in the non-equilibrium superconducting state even though the gap vanishes. This, in turn, greatly modifies the tunneling characteristics of the device, bringing them far from that which would obtain at equilibrium. Still further, heavy injection can lead to a device output >2A, which can be used to drive another similar device since it will produce energetic quasi-particles in the driven device.
- In Fig. 1.1, a superconducting-insulator-normal conductor structure is shown. A
tunnel barrier 10 is located between the normal conductor N and asuperconductor S. Barrier 10 can be, for example, an oxide of the normal metal N. A current IT tunnels throughbarrier 10 and produces a voltage VT across the structure. The voltage V12 is a voltage measured betweenpoints - This SIN tunnel junction is a means by which quasi-particles can be injected into superconductor S. This is a known structure and is used to illustrate the injection of quasi-particles.
- The tunneling current IT creates an excess quasi-particle number density
- Fig. 1.2 shows the dependence of the gap A (normalized) on the excess quasi-particle density (normalized) at T=0°. The
top curve 12 is derived in accordance with the known BCS(T*) model, while thebottom curve 14 is derived from the known Owen-Scalapino µ* model. - When the injected power density increases to Pc=JTcVTc' the voltage V12 makes a transition from zero to a finite value. This is indicated in Fig. 1.3, which is a plot of the tunneling current IT as a function of the voltages VT and V12. Thus,
curve 16 applies when P<Pc, whilecurve 18 is for P>Po. The threshold power density Pc and its dependence on injeeted quasi-particle energy directly affect the characteristics of the Quiteron, since they determine the gain, power dissipation, and speed of this device. - The critical injection current density JTc can be much smaller than the critical current density Jc of the superconductor S. This distinguishes the non-equilibrium mechanism of the Quiteron and the equilibrium mechanism as far as gap suppression is concerned. The ratio Jc/JTc is the absolute upper limit of the Quiteron gain. Another distinguishing factor is the speed of the non-equilibrium mechanism, which is much faster than that of the equilibrium mechanism.
- As the external excitation is removed, i.e., the quasi-particle injection is stopped (IT→0), and the gap recovers its equilibrium value A in a time (relaxation time) governed by several mechanisms. These mechanisms include inelastic scattering, recombination of holes and electrons with the emission of phonons at nw>2A, phonon escape from the superconductor, and quasi-particle diffusion. These mechanisms also influence the critical power density.
- At the critical excess quasi-particle density nc(Pc), the gap A will vanish. The threshold power density is therefore the density which is required to observe the voltage V12· It should be noted that in the transistor of U.S. 4,157,555, the gap A remains at its equilibrium value or very near to it (i.e., the change in A is «1 %) and the voltage V12 is not observed. The output voltage of that device is intended to be <2Δ and its current-voltage characteristics remain substantially the same. In turn, this means that a switching device useful for logic applications cannot be obtained, nor can an amplifier having linearity with large gain. The following expression has been derived for the threshold power density Pc at T=0:
- It is known that Pc does not change as a function of injected voltage VTc, which is a valid assumption for VTc>>2△. This has been experimentally verified in lead-tin junctions over a wide voltage range. The dependence of Pc on the injected quasi-particle energy is expected to rise very sharply at the normalized quasi-particle enerqy
tunnel barrier 20 therebetween are shown in Fig. 2.1. - The film S2 is thin enough so that its gap A2 vanishes at Pc, while superconductor S1 is much thicker and consequently is not affected by Pc. The sharp rise of Pc at ε~1 is expected to occur due to the very sharply peaked densities of states at the gap edges of both materials. At s<1, Pc saturates to the thermal power density PT=Pc(0), because the only phonons that will destroy superconductivity are those provided by heating the film to Tc. The other extreme, ε>1, exhibits a value Pc(∞) which takes the value given by Equation (1). In this case, the injected quasi-particles recombine with the emission of phonons at energies greater than 2A2, which in turn break pairs in a time faster than they can escape from the film. At T=0, Pc is expected to rise sharply to a value PT(0)=25.6 d/T watt/cm2, for lead, where d is in lengths of nm and T is in pico-seconds.
- The discovery of the Pcvs. s threshold relationship shown in Fig. 2.2 led to the recognition that a digital switch with an inversion property could be conceived in which the output would be sufficient to drive other similar switches, and which would exhibit high gain. This invention restricts the current density of the injector such that Pc is reached at output voltages of approximatley 2(△2+△3). This determines the choice of device materials, film thickness, and operating temperature in order to realize an inverting device having sufficient gain and capable of switching similar devices. The Pcvs. s relationship is not suggested from the prior art, and particularly from U.S. 4,157,555. The device of that patent is restricted in output voltage, current levels, and choice of materials such that the gap is not affected. Therefore, the advantages of high gain, etc. are not possible.
- The importance of the power in the Quiteron action is that it is a threshold quantity, i.e., when Pc is reached the gap vanishes. Large currents alone, or large voltages alone, do not necessarily cause the gap to vanish. An excessive amount of current will be required if an equilibrium phenomenon is relied upon. In this situation, the critical current of the film itself must be exceeded and no pair breaking will occur until the critical current is exceeded. However, in the non-equilibrium situation caused by heavy injection of high energy quasi-particles, less power is required to have the gap A to vanish to 0 in accordance with the relation shown in Fig. 2.2. The amount of voltage which is applied is sufficient to observe pair breaking, the minimum voltage being 2Δ. The applied voltage is chosen to be sufficient to sustain the gap at △=0 as long as that is required. Thus, voltages are applied such that Cooper pairs break, and so that the gap vanishes. Relatively lower injected current and relatively lower voltage [≳(Δ1+Δ2) result in small power capable of making Δ→0. The energy of the injected quasi-particles and the number injected is sufficient to make a significant change in the gap Δ. Non-equilibrium phonomena are utilized, and the output voltages are sufficiently high that similar Quiteron-type devices can be driven.
- Fig. 3.1 shows a structure comprising first and second superconductors S, and S2, having gaps Δ1, and Δ2, respectively, and separated by a
tunneling barrier 22. A tunnel current I flows through the tunnel barrier, and a voltage V is established across the barrier. - The current-voltate (I-V) characteristics of the tunnel junction in Fig. 3.1 are shown in Fig. 3.2. The
solid curve 24 is that due to an equilibrium state where P<<Pc. As will be more apparent later,curve 24 must be one in which a current transition occurs at Δ1+Δ2. This feature is required for the equilibrium I-V characteristic of the acceptor junction. The dashedcurve 26 corresponds to that which occurs when P is greater than Pc (i.e., Δ2→0). The modifiednon-equilibrium curve 26 resembles that of a normal-superconductor junction which has a sharp current rise at V=Δ1. - When the threshold power density Pc is achieved, the I-V characteristic changes from
curve 24 tocurve 26. When P is removed, the tunnel device switches back to theI-V curve 24. The ability to switch back and forth betweencurves - 1. Creation of quasi-particles in electrode S2 by photons having energies larger than 2A2 such that Cooper pairs can be broken.
- 2. Phonon beams with hω>2Δ2.
- 3. Injection of quasi-particles by means of another tunnel junction.
- 4. Electron, ion, and neutron beam.
- In Fig. 3.1, the
arrows 28 represent the application of injected quasi-particles into superconductor S2, as for example by photon beams. - Quiteron embodiments (Figs. 4.1, 4.2, and 4.3) The Quiteron devices of Figs. 4.1 and 4.2 illustrate embodiments wherein the injection of quasi-particles is by means of another tunnel junction, which will be called the injector junction. As an alternative the injector could be a weak link device. In Fig. 4.1, the output junction (acceptor junction) is comprised of a single junction and the device is termed a single junction acceptor (SJA). In Fig. 4.2, the output junction is comprised of two junctions in series and the device is called a double junction acceptor (DJA). As will be seen later, the design of the DJA has advantages over the SJA design, particularly in the choice of the electrodes, and in the greater output voltage that can be obtained.
- In Fig. 4.1, the
Quiteron device 30 is comprised of three superconducting films S1, S2, and S3, having gaps Δ1, Δ2, and Δ3, respectively. As will be seen later, maximum discrimination betweenbinary - Middle electrode S2 is made thin enough so that its gap Δ2 can be suppressed to zero when the threshold power density Pc is exceeded. Films S1 and S3, on the other hand, are thick enough to prevent them from going normal when Pc is exceeded.
- A
tunnel barrier 32 is formed between S1 and S2, and asecond tunnel barrier 34 is formed between S2 and S3. Tunnel junction 32 is termed the "injector" junction since its function is to inject quasi-particles into superconductor S2. Junction 34 is termed the "acceptor" junction since its tunneling characteristics will be modified depending upon the injection of quasi-particles into S2. - In the operation of this device S1 can be a non-superconductor (i.e., normal) or it can go normal, without adversely affecting device operation. S2 must be a superconductor whose gap can be drastically reduced, and is sufficiently thin that a reasonable input power can be used to do that. S3 is preferably a thicker superconductor, since the device is faster if only S2 has its gap drastically changed when P>Pc is applied. If S3 stays superconducting, then two distinct states are achieved and only S2 has to return to its original Δ2 state when the input power is removed. This means that switching between the different output states will be faster.
- In Fig. 4.1, electrode S3 is a continuous electrode across the entire lateral extent of
barrier 34. Thus, the device is a single junction acceptor. - The injector and acceptor junctions are biased with DC current sources Ii and Ia, and a load RL is connected across the acceptor junction formed by electrodes S2 and S3 having
tunnel barrier 34 therebetween. The voltage across RL is VL. - Fig. 4.2 shows a double junction acceptor in which two acceptor junctions are in series. In this device, superconductors S1, S2, and S3 are provided having gaps Δ1, Δ2, and A3, respectively. S1 and S2 are separated by a
tunnel barrier 36, while superconductors S2 and S3 are separated bytunnel barrier 38. S2 is sufficiently thin that its gap can be suppressed to zero when Pc is exceeded, while films S, and S3 are sufficiently thick that they will not go normal when Pc is applied. The same considerations with respect to S1, S2, and S3 described for the SJA structure of Fig. 4.1 apply to the structure of Fig. 4.2. The DC bias sources Ii and Ia are also the same as those described with respect to Fig. 4.1, as is the linear load RL. - In the embodiment of Fig. 4.2, the acceptor junction is comprised of two tunnel junctions in series. One of these is formed between S3 and S2, while the other is formed between S3' and S2. As was previously mentioned, the use of at least two junctions in series for the acceptor junction leads to a Quiteron whose output voltage is >(Δ1+Δ2) and hence it can drive another Quiteron device. In the SJA, greater constraints exist on the electrode materials in order to achieve the same results.
- Fig. 4.3 shows the current-voltage characteristics (I-V) of the Quiterons of Figs. 4.1 and 4.2 across the linear resistive load RL. When the injector bias Ii=0, the load intersects the
solid I-V curve 40 at a load voltage VL 1=2(Δ2+Δ3). This represents the binary state "1" of the output current IL through RL. - If the injection current Ii is increased such that the injected power Pi=IiVi exceeds the threshold power PcX area, the gap A2 will vanish and the load line will intersect the dashed
curve 42 at a load voltage VL°=2Δ3. This is the "0" binary state of the Quiteron. Thus, the Quiteron can be used as an inverter, which is a feature exhibited by transistors. It operates in a non-latching mode because it can be switched back to the "1" state when Pi→0 (i.e., it does not stay in the VL° state when Pi=0). - In the practice of this invention, it is important that the acceptor junction exhibit an equilibrium I-V curve which has a current transition at V=A2+A3, i.e., it must exhibit a two gap characteristic. For this reason, the acceptor tunnel barrier is generally an insulating material, such as an oxide of S2. The injector tunnel barrier can be more arbitrarily selected and can be, for instance, a normal metal, a semi-metal, an insulator, a semi-conductor, etc. Although layered structures have been shown for the SJA and DJA devices, other geometries can be used. For example, the acceptor junction and/or the injector junction can be an edge junction.
- For a useful device, it must be possible to apply the output of one Quiteron to drive another Quiteron, so that circuits comprising a plurality of Quiterons can be formed. It is a primary object of this invention to describe such a device, in order to provide useful logic and amplifier circuits.
- Fig. 5.1 shows two directly coupled DJA Quiterons. In this circuit, the output of the
first Quiteron 44 is used to drive the input of thesecond Quiteron 46. The injector junction ofQuiteron 46 represents a nonlinear load to the output ofQuiteron 44. As will be more apparent with respect to Fig. 5.2, this provides a nonlinear load line leading to a circuit having very good discrimination between different output states. - In more detail,
Quiteron 44 is comprised of electrodes Sl, S2 , and S3, having superconducting gaps Δ1, Δ2, and A3, respectively.Tunnel barrier 48 is provided between S, and S2, whiletunnel barrier 50 is provided between S2 and S3, as well as between S2 and S3'. Typically, S3 and S3' are comprised of the same material. -
Quiteron 46 is also comprised of three electrodes having the same designations Sl, S2, and S3. The superconducting gaps are Δ1 Δ2, and Δ3. Superconductor S3' is typically the same as S3, and therefore also has the gap Δ3.Tunnel barriers tunnel barriers - In a typical circuit, the materials comprising electrodes of
Quiteron 46 will be the same as the material providing the electrodes inQuiteron 44. Additionally, thetunnel barriers tunnel barriers - A current Ii is provided to the injector of
Quiteron 44, and is designated as input A. Since the Quiteron is an inverter, the output ofQuiteron 44 is A. Theacceptor junction 50 ofQuiteron 44 is biased with DC current Ia1 while theacceptor junction 54 ofQuiteron 46 is biased with DC current Ia2. A load 56 is provided acrossQuiteron 46. - It is possible for
Quiteron 44 to driveQuiteron 46 since the gain ofQuiteron 44 is sufficiently high that its output power in the "1" state, P1, exceeds the threshold power ofQuiteron 46, while that of the "0" state, Po, does not.Quiteron 46 represents a nonlinear load toQuiteron 44 and this results in a significant increase in the discrimination ratio D≡P1/P0 between the "1" and "0" states. In order to take full advantage of this nonlinear loading, the condition - The discrimination ratio is then given by:
Io≃0.7(Δ2+Δ3)/RNN. RNN is the normal tunnel resistance, and Vm describes the quality of the junctions which ranges between 25 mV and 50 mV for lead alloy junctions. The leakage current - Fig. 5.2 shows the I-V characteristics of the
acceptor junction 50 with the nonlinear load represented byQuiteron 46. Thesolid curve 58 is the curve followed when the input excitation toQuiteron 44 is less than the threshold power density, while the dashedcurve 60 is the I-V characteristic when the threshold power density ofQuiteron 44 is exceeded such that Δ2 vanishes. The load line has two portions: Afirst portion 62 representing a high resistance and asecond portion 64 representing a lower resistance. The binary "1" state occurs where the load line intersectssolid curve 58, while the "0" state occurs where the load line intersects dashedcurve 60. - Fig. 6 is a I-V plot of the acceptor characteristics of
Quiteron 44 when the load for this Quiteron is a nonlinear load, and in particular isQuiteron 46. Fig. 6 is essentially the same as Fig. 5.2, except that it is used to define the parameters which are used to derive the expressions for the discrimination ratio (Equation 2) and for the gain of the device (Equation 7). As was described with respect to Fig. 5.2, 2A3 is less than Δ1+Δ2, which in turn is less than 2(A2+A3). The binary "1" state is - Fig. 7 is a plot of the threshold power density Pc as a function of the normalized quasi particle energy
Quiteron 44. The binary "0" state is atEquation 2 for the discrimination factor D has to be multiplied by the ratio Pc(0)/Pc(∞) to obtain - For Pc(0)/Pc(∞)=23.8, and for Vm=30mV, Δ1=Δ2=1.2Δ3=1.36mV, D=1932.
- The fact that discrimination occurs between power levels instead of between either voltage alone or current alone is a unique feature of the Quiteron. This has a significant impact on noise immunity, which can be described by a noise discrimination ratio DN:
- The Quiteron devices do not require the .presence of Josephson tunneling current through the tunnel barriers. Therefore, resonance problems do not arise and these devices are not ultrasensitive to trapped flux. However, Josephson current can be present, as will be explained with respect to the embodiments of Figs. 21.1, 21.2, 22, and 23.
- The power gain G of the Quiteron is the ratio of the output power across the load
- The acceptor current density ja and the injector current density ji are measured at the same voltage (A2+A3). Equation 6 indicates that by increasing the current density ratio it is possible to obtain G»1. It also indicates that the gain does not depend on the lateral dimensions of the Quiteron, but instead depends upon the tunnel barrier thicknesses.
- Hence, the Quiteron device is potentially miniaturizable. For example, for ja/ji=100, the tunnel barrier thicknesses of the acceptor and injector barriers differ by merely 3 angstroms. In the Quiteron, the ratio Ja/Ji is not arbitrarily chosen; instead, ji is chosen such that
- Gains of 10 are easily realized using lead based alloy electrodes and an acceptor junction area which is 1/2 the injector junction area, and for ja=14,600 A/cm2. If the ratio ja/ji is increased, more gain is possible. Since tunnel current densities as high as 2x105 A/cm2 have been realized in lead alloys, this seems possible. However, there are fundamental limitations wihch fix the maximum gain Gmax. For example, it is desirable to keep ja low enough that
- It has previously been mentioned that it is advantageous to fabricate the acceptor from at least two junctions in series rather than from only one junction (SJA). One reason for this is the following. More flexibility is obtained in choosing the superconducting materials used for the electrodes in the case of a DJA than for an SJA because of the conditions imposed on the gaps. These conditions are
Equation 3, and in thenoise immunity Equation 4, as well as in the expression for maximum gain (Equation 7). In contrast with this, it is not possible to benefit from the ratio Pc(0)/Pc(∞) when Δ1=Δ2=Δ3 in the single junction acceptor Quiteron. Still further, the output voltage of the DJA is 2(Δ2+Δ3), while for the SJA it is (Δ2+Δ3). The SJA benefits from the ratio Pc(0)/ Pc(∝) if the constraint - Since the gain as given by Equation 6 does not depend upon the lateral dimensions of the Quiteron, advanced lithographic techniques and processes can be used to miniaturize these devices. At small line widths, the gain will be affected by the quasi-particle diffusion length. Since the operation of this device is based on overinjection of quasi-particles, the diffusion length decreases to about 100 nm, which is assumed to fix the smallest line width.
- The non-equilibrium phenomena used to change the state of this device provides quite high switching speeds. In particular, superconductors having high Tc, and their alloys, should have the shortest quasi-particle relaxation times. The most dominant relaxation processes are recombination to form a bound Cooper pair with the emission of a phonon, and inelastic scattering with phonons. The recombination times and inelastic times become shorter at higher energies, which further proves the significance of the double junction acceptor embodiment. With overinjection of quasi-particles, there is an enhanced probability for recombination, which in turn produces more phonons to engage in the inelastic scattering process. If these phonons are sufficiently energetic, they will break additional Cooper pairs, to create even more quasi-particles. This helps to sustain the gap at A=0.
- From a power dissipation point of view, it appears that very low switching energies are required to change the state of the Quiteron. The absolute power dissipation per bit drops rapidly as 1/WZ for these devices, where W is the linewidth. For example, if a device has a switching speed less than about 100 psec., then at W=1 micrometer, approximately 6×10-18 joules (gain=1) will be required to switch from one binary state of the device to the other.
- The three electrode devices shown herein use a very thin superconductor S2 whose gap is modified by overinjection of quasi-particles. Electrical contact must be made to this very thin electrode. Additionally, the electrodes S1 and S3 are thicker, and this means that the thin middle electrode S2 has to step down a thick electrode Sl. Fig. 8 shows a scheme for making electrical contact to the thin electrode S2.
- In more detail, Fig. 8 shows the electrodes Si, S2, and S3, which are separated by the
injector barrier 66 andacceptor barrier 68. As is apparent, the injector.junction 66 is thinner in the region where the Quiteron action occurs, but is thicker in the region where electrical contact is to be made to S2. The thickmetallurgical layer 70 is used to contact S2. If desired,acceptor barrier 68 can be formed from an insulator which is thicker in the region outside of electrodes S3 in the same manner that the injector barrier is thicker outside the device area. This will further ensure that there is no undesired electrical contact betweenconductor 70 and S3. - Fig. 9 shows a top view of a layout for a double junction acceptor device such as that in Fig. 8. The bottom electrode is Si, while the thin middle electrode is S2. The top electrode is S3. The additional
thick metallurgy layer 70 is used to make contact to thin electrode S2 and is shown in the side view of Fig. 8. Theacceptor junction 68 is a duoble junction, while theinjector junction 68 is formed between S1 and S2. The injector current flows intocontact strip 70 and out of S1. Layer 72 is an insulating layer which electrically insulates S1 from S3. It is, for example, a layer of SiO. - In Fig. 9, W is the minimum linewidth. Thus, this device layout occupies an area 21 W2. This represents an improvement in density over the holey and stepped interferometers of the prior art. One disadvantage of the Quiteron is in the required control of the current densities ji and ja to within reasonable tolerances. However, this disadvantage may be rendered less severe due to the availability of gain, large discrimination factor, and high noise immunity.
- This device has applications in digital: systems, analog systems, and in the study of the physics of non-equilibrium superconductivity. The technology necessary to make superconductive tunneling devices, such as Josephson tunneling devices, can be used to fabricate circuits of Quiterons. The following sections will describe the use of these devices in logic circuits, and as analog amplifiers.
- Quiteron logic circuits (Figs. 10.1, 10.2, 11.1, 11.2, 12-18) 1 As will be apparent from the following, NOR logic circuits can be built using these devices. This provides advantages in fabrica- tion/density/scaling over the Josephson device circuits, in which AND, OR, and INVERT blocks are needed to design complete logic circuits. Because the Quiteron is an inverting device, a NOR block can easily be designed, and this can be used to build a complete family of logic circuits. For this application the SJA configuratiion. is more . suitable than the DJA configuration. However, the DJA configuration has an advantage from the fabrication and metallurgical points of view. These configurations can be combined in a single circuit, as will be apparent from the full adder of the Fig. 17.
- The ratio η=P(0)/P(∞)>>1 has been mentioned to enhance the maximum gain, the discrimination ratio, and the noise immunity. In order to take advantage of η, certain constraints are applied tc Δ1 A2, and A3 for both the SJA and DJA devices For the SJA device
-
- Fig. 10.1 shows an SJA configuration, while Fig. 10.2 shows the I-V characteristic. The ratio of the 1 voltage state to the 0 voltage state is
injector junction 74 is located between S, and S2 while theacceptor junction 76 is located between S2 and S3. Anonlinear load 78 is connected across the acceptor junction and can be, for example, another SJA device. -
- Fig. 11.1 shows a DJA configuration, while Fig. 11.2 gives the I-V characteristics and indicates the voltages when the device is in its 0 and 1 states. The
injector junction 80 is located between S1 and S2, while theacceptor junction 82 is located between S2 and S3. Anonlinear load 84, preferably another DJA configuration, is connected across the acceptor. Ii provides DC bias to the injector junction, while Ia provides DC bias to the acceptor junction. - In order to take advantage of the discrimination when a nonlinear load is used,
- Fig. 12 shows DJA devices powered by current sources Ia1, Ia2, and la3 in order to obtain a fan-out of four. The DJA Quiterons are shown schematically, and are labelled Q1, Q2, Q3, Q4, and Q5. The loads L2, L3, L4, and L5 are connected across devices Q2―Q5, respectively.
- It would be ideal to string DJA devices in series by means of a single current source. However, because of undesirable current leakage paths, a multiplicity of current sources is required, as shown in Fig. 12. Each device is restricted to have a fan-out of two for each different current source. For example, device Q1 drives the two devices Q2 and Q3 powered by 1a2, and drives devices Q4 and Q5 powered by Ia3, thus providing a total fan-out of four. This powering scheme is of limited utility because only simple logic functions can be implemented. An undesirable number of current sources is necessary in order to implement more complex functions.
- The SJA device configuration avoids the isolation problems encountered by the DJA device. This occurs because the center electrode is grounded, and hence current return paths to the input are eliminated. For example, Fig. 13 illustrates a parallel powering scheme which requires a single voltage bus VS and a power resistor Rs for each device. This scheme dissipates more power than a series connected DJA configuration, but takes better advantage of Quiteron density. The power resistor Rs can be made with a suitable tunnel junction, especially since the supply voltage VS is a DC voltage.
- In Fig. 13, device Q1 has an injector input A, and will provide an acceptor output A when A exceeds the threshold power density. Only when input A is present to device Q2 will the outputs A be realized from Q2.
- A is also an input to device Q3. Its output will be A, which is provided to the acceptor electrode of device Q4. Only when input B is present to Q4, will the
output 8 be provided from Q4. Thus, the presence of A to Q3 at the same time that B is provided to Q4 is required to produce the output A B. - Fig. 14.1 shows a three input NOR circuit with Quiterons Q,, Q2, and Q3. A single voltage bus delivers the power VS through a resistor Rs. Thus, acceptor current Ia flows to each of the acceptors of devices Q1―Q3. Input A is provided to the injector of Q1, input B is provided to the injector of Q2, and input C is provided to the injector of Q3. The presence of any input A, B, or C will provide an output A or B or C, respectively.
- The I-V characteristics of the three input NOR circuit of Fig. 14.1 are shown in Fig. 14.2. When all the inputs A, B, and C are in the "0" state, devices Q1―Q3 assume the equilibrium characteristics given by the
solid curve 86, and nearly all of the current Ia is delivered to the load (not shown). This load is the injector nonlinear characteristics of the fan-out Quiterons. If one or more of the inputs A, B, C are in the "1" state, the non-equilibrium characteristics represented by the dashed curves 88, 90, and 92 are developed, causing the load voltage to switch to A3 and the current to diminish to Δ3/Rj (load).Curve 88 appears when one of the inputs is present whilecurve 90 appears if two inputs are present. Dashed curve 92 appears when all three inputs are present. - Fig. 15.1 shows a multi-injector NOR configuration which is nearly identical to the three input NOR circuit made of discrete Quiterons, as shown in Fig. 14.1. In the circuit of Fig. 15.1, a single acceptor is provided, while three separate injectors are provided, having inputs A, B, and C. A voltage bias VS is provided through resistor Rs to the acceptor junction. When any of the inputs A, B, or C is present, the output A or B or C, respectively, will be provided.
- The difference between the circuits of Figs. 15.1 and 14.1 is evident in the I-V characteristics. The characteristic of Fig. 15.2 shows an additional current transition la at V=Δ2+Δ3. This reflects the fact that the ratio of acceptor area Aa to the injector area Ai is larger than unity for the multi-injector configuration of Fig. 15.1. Therefore, this type of configuration may have a gain advantage as well as a density advantage over the single injector configuration of Fig. 14.1.
- The I-V characteristics of Fig. 15.2 are otherwise the same as those of Fig. 14.2. Thus, solid curve - 94 is the curve followed when no input is present, dashed
curve 96 is the curve followed when one input is present, dashedcurve 98 is the one followed when two inputs are present, and dashedcurve 100 is the curve followed when all three inputs are present. - Fig. 16 shows a four-bit decoder using parallel powered NOR blocks. The circuit is comprised of the single injector SJA devices Q1, Q2, Q3, Q4, etc. Multi-injector SJA devices Q5, Q6, Q7,...,Q20 are also provided. The acceptor outputs of Q5―Q20 provide the decoder outputs.
- The decoder structure of Fig. 16 is similar to other decoders known in the art. Depending upon the injector inputs A1, A2; A3, and A4, devices Q1―Q4 will provide outputs A1―A4. The outputs of Q,-Q4, as well as the inputs A1―A4, are provided in different combinations to the multiple injectors of Q5―Q20. As is well known in the decoder art, depending upon the combinations of the four inputs, 16 outputs 01-016 can be obtained.
- Fig. 17 shows a one-bit full adder using both SJA and DJA device configurations. The design of this circuit is similar to the design of other full adders and provides as an output the complement Snof the sum signal and the complement Cn of the carry signal. The adder is comprised of three SJA devices Q1―Q3 at the circuit input, and three double injector Quiterons Q5―Q7. Additionally, five triple-injector Quiterons Q8―Q12 are used as well as one quadruple injector Quiteron Q13. The acceptor junctions of all the Quiterons are provided with the same supply voltage VS through resistors Rs.
- Devices Q1―Q3 provide the complements of the inputs Xn, Yn, and Cn-1The outputs from the acceptors of Q1―Q3 are applied to the injector junctions of Quiterons Q9―Q11. Devices Q5―Q8 receive as injector inputs various combinations of the circuit inputs Xn, Yn, and
C n-1 The acceptor outputs of devices Q8―Q11are applied as injector inputs to Q13 to develop the complement Sn, while the acceptor outputs of devices Q5―Q7 are the injector inputs to Q12, to provide the complementC n of the carry signal. - Since a Quiteron is an inverter device, it can be utilized to design a flip-flop circuit, such as the NOR flip-flop shown in Fig. 18. A circuit of this type can be used to provide memory latches, as well as memory arrays when an appropriate selection scheme is developed.
- The flip-flop is comprised of two double injector Quiterons Q1 and Q2 which are cross-coupled to one another. A DC bias Vs is applied to the acceptor junction of each device Q1 and Q2 through resistances Rs. A SET signal S is applied to one injector input of Qi, while a RESET signal R is applied to one injector input of Q2. The other injector input of Q1 is connected, via
line 102, to the acceptor output of Q2, while the second injector input to Q2 is connected to the acceptor output of Q1 viaelectrical connection 104. The operation of the flip-flop is the same as conventional flip-flops, a true signal T and a complement signal C being provided at the acceptor outputs of Q1 and Q2, respectively. - The Quiteron structure can be used to provide analog amplification of the input signals to its injector junction. For this use, it is desirable to change the superconducting gap drastically, but it is not necessary to reduce the gap to zero. In fact, if the gap is reduced to zero, a linear output will not be obtained. Since linearity is important in an amplifier, the input signal should be less than that which would cause the gap of the superconductor into which quasi-particles are injected from vanishing.
- Fig. 19 shows the current-voltage characteristics of a Quiteron device used as an analog amplifier. The
solid curve 106 is the I-V characteristic when the gap Δ=Δ2. That is, this is the equilibrium condition. The dashedcurve 108 is the curve which would result if Δ3+Δ2→0. As mentioned, a threshold type of operation is necessary for digital switch applications, but this threshold may not be reached when amplification is desired. - Depending upon the magnitude of the input signal to the injector, a family of
curves - In operation, the amplifier can be biased at a voltage V1, which corresponds to a
power input P 1/2 Pc, and the presence of an input signal causes an output which is determined by the linear voltage of theload line 124. Thus, output voltages from V2 to V3 can be obtained depending upon the input signal magnitude. Large current amplification can be obtained in this way, in contrast with the amplifier of U.S. 4,157,555 in which P«Pc. It is not possible to obtain large signal gain in that prior art transistor. In contrast with this, the superconducting gap is drastically changed to provide a full family of curves 110-122 in order to provide a large gain. - Although the Quiteron embodiments presented here have been described in detail with respect to the relationships between the superconducting gaps of the different electrodes, it is readily apparent that Quiteron-type devices can be built without maximizing the discrimination ratio, gain, and noise immunity. Examples have been shown for the types of superconductors and their alloys which yield different superconducting gaps. Still further, it is possible to use normal (i.e., non-superconducting) materials for either the top or bottom electrode, or both. However, it is desirable that S3 be a superconductor since this will provide the best gain, discrimination, etc. However, by choosing appropriate superconductors, the Quiteron device characteristics can be enhanced as described previously.
- In the preferred embodiments, injection of quasi-particles from a tunnel barrier provides over injection of quasi-particles into the superconductor whose gap is to change. This type of injection input can be modified to use a combination of inputs, such as electrical inputs and photon beams. Thus, a Quiteron can be fabricated in which an electrical signal is applied to the injector junction as well as a photon beam. Also, the input excitation can be due to only the photon beam in order to provide a full optical transistor.
- The thicknesses of the various electrode layers are chosen with respect to the device characteristics that are required. In particular, the thin superconducting electrode S2 whose superconducting gap must be drastically altered should be sufficiently thin that this can be achieved without the application of unduly large signals. A representative thickness for the electrode S2 is 50-100 nm. This is dictated by the fact that the threshold power density Pc must be reached of reasonable power levels. The design proceeds as follows: an S2 thickness of, for example, 50 nm is chosen to obtain Pc=1 watt/cm2 for lead at
- The thicknesses of the tunnel barriers are chosen · to provide appropriate ratios of the current densities j. and j,. While the presence of Josephson tunneling current through either of the junctions is not required for operation of this device, tunnel barriers can be provided where Josephson current is present. This current could be used to provide a device with three states, which is capable of latching. Representative values for the thicknesses of the injector and acceptor tunnel junctions are such that a current density ratio ja/ji≳100 is obtained. Devices employing Josephson currents, and devices using different biasing will now be described.
- Quiteron with different biasing (Figs. 20.1 and 20.2)
- For illustrative purposes, a SJA device is shown in Fig. 20.1, while its I-V characteristic is shown in Fig. 20.2.
Quiteron 126 is comprised of three electrodes S1' S2, and S3, theinjector junction 128, and theacceptor junction 130.Current source 132 provides injection current li while theacceptor junction 130 is biased bycurrent source 134, which produces current la. In contrast with the previously shown biasing schemes,load 136 is connected across S1 and S3 in this embodiment. - In Fig. 20.2, the
equilibrium curve 138 has a current transition at Δ1+2Δ2+Δ3, in contrast with the usual equilibrium curve for a SJA device, which has a current transition at Δ2+Δ3. Thus, a greater diacrimination can be obtained between the "1" and "0" states. Here, the total equilibrium voltage curve depends upon the voltages across both the injector and acceptor barriers. For this reason, a larger voltage swing can be obtained than with the other SJA embodiments. However, the voltage swing is not necessarily greater than that which is obtained with a DJA structure, and the biasing technique of Fig. 20.1, is not as satisfactory from hhe standpoint of isolation between the output and the input. - These embodiments describe switching between multiple output states, at least one of which is a Josephson state exhibiting 0-voltage current. One of these embodiments (Fig. 23), three distinguishable output states are provided.
- In more detail, Fig. 21.1 shows a SJA structure including electrodes S1, S2, and S3. S2 and S3 are superconductors, while S1 can be a superconductor or a normal material.
Injector tunnel barrier 140 is located between S1 and S2, whileacceptor tunnel barrier 142 is located between S2 and S3. Alinear load 144 is connected to the acceptor electrode S3. Injector current li flows into S1 and out of S2, while the acceptor is biased by current la which flows into S3 and out of S2. Ifbarrier 142 is sufficiently thin, a zero-voltage current will tunnel through this barrier in accordance with the Josephson effect. As an alternative, the acceptor can be comprised of a weak link if one of the device output states is a zero-voltage current. For this embodiment the term "tunnel junction" is meant to include a weak link device as well as one having electrodes separated by a tunnel barrier, i.e., any device exhibiting a Josephson current. - The I-V characteristics of the Quiteron of Fig. 21.1 are shown in Fig: 21.2. Different curves are obtained for different values of the Stewart-McCumber hysteresis parameter β.This parameter is known in the art and is one which measures the amount of hysteresis obtained when the current-voltage characteristics are plotted. A junction will exhibit hysteretic effects when β»1 and will be non-hysteretic for values of (β≤1. As is known, β is given by the following expression:
tunnel barrier 142, R is the total junction resistance, and Ci is the junction capacitance. - Curve 146 is the equilibrium curve of the Quiteron where β«1 (i.e., a hysteretic junction) while
curve 150 represents the equilibrium state when β«1. When P>P is applied, the characteristic switches to that depicted bycurve 148 orcurve 152, depending on the amount of power overdrive. - In Fig. 21.2, two load lines having slopes Rl1 and RL2 are shown. A zero-voltage Josephson state can occur when Josephson current tunnels through the acceptor junction. This occurs for (β«1. As P increases to Pc, the device switches from a Josephson state to a state given by the intersection of the load line and
curve 152. Thus, this Quiteron is non-inverting, and non-latching. - Fig. 22 shows the I-V characteristics of a Quiteron exhibiting a 0-voltage Josephson state, where the load is now a nonlinear load, such as that represented by another Quiteron device.
Equilibrium curve 154 represents the I-V characteristic for a junction having β»1 (hysteretic), while thesolid line curve 156 represents the equilibrium curve for a junction in which (β«1 (non-hysteretic).Curves curve 160 is the I-V characteristic when a non-hysteretic junction has Δ2=0.Nonlinear load line 162 intersectscurve 156 at a point corresponding to a binary "0", and intersectscurve 160 at a point representing a binary "1" state. - The device of Fig. 22 is non-inverting, and non-latching.
- Fig. 23 shows an I-V curve for the Quiteron of Fig. 21.1 biased to have three states, where one state is a Josephson zero voltage state. The Quiteron of Fig. 21.1 has an
equilibrium curve 164 and, if the acceptor barrier is sufficiently thin and there is magnetic shielding, a Josephson current can flow through the acceptor barrier. For the Quiteron of Fig. 23, (β»1, and the load is linear, as shown bylinear load line 166.Curve 168 represents the situation when P>Pc. - When P=0, and I<lo (lo is the Josephson critical current), a Josephson zero-voltage state (first state) can exist. When the Quiteron is switched by an input power P>Pc, Δ2 vanishes and another state (second state) exists at the intersection of the load line and
curve 168. Now, when the power is removed, the Quiteron switches to a third state represented by the intersection ofload line 166 andcurve 164. In order to retrieve the first state, the current la has to be removed and applied again. Therefore, this is a non-inverting and latching device.
Claims (8)
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
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DE8181110277T DE3175179D1 (en) | 1981-12-09 | 1981-12-09 | Superconducting tunnel junction device |
EP81110277A EP0081007B1 (en) | 1981-12-09 | 1981-12-09 | Superconducting tunnel junction device |
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EP81110277A EP0081007B1 (en) | 1981-12-09 | 1981-12-09 | Superconducting tunnel junction device |
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EP0081007A1 EP0081007A1 (en) | 1983-06-15 |
EP0081007B1 true EP0081007B1 (en) | 1986-08-20 |
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EP81110277A Expired EP0081007B1 (en) | 1981-12-09 | 1981-12-09 | Superconducting tunnel junction device |
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DE (1) | DE3175179D1 (en) |
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4780749A (en) * | 1986-07-01 | 1988-10-25 | Hughes Aircraft Company | Double barrier tunnel diode having modified injection layer |
EP0407166A2 (en) * | 1989-07-05 | 1991-01-09 | Canon Kabushiki Kaisha | Light detecting device and light detection method |
US5296721A (en) * | 1992-07-31 | 1994-03-22 | Hughes Aircraft Company | Strained interband resonant tunneling negative resistance diode |
US6365912B1 (en) | 1998-06-17 | 2002-04-02 | Isis Innovation Limited | Superconducting tunnel junction device |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
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DE4010489A1 (en) * | 1990-03-31 | 1991-10-02 | Dornier Luftfahrt | SUPRALOCIAL ELEMENT |
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US4157555A (en) * | 1977-11-07 | 1979-06-05 | The United States Of America As Represented By The United States Department Of Energy | Superconducting transistor |
-
1981
- 1981-12-09 EP EP81110277A patent/EP0081007B1/en not_active Expired
- 1981-12-09 DE DE8181110277T patent/DE3175179D1/en not_active Expired
Non-Patent Citations (1)
Title |
---|
Physical Review B, vol. 4, no. 7, 1 oct. 1971, L.R. Testardi, "Destruction of superconductivity", pp. 2189-2195 * |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4780749A (en) * | 1986-07-01 | 1988-10-25 | Hughes Aircraft Company | Double barrier tunnel diode having modified injection layer |
EP0407166A2 (en) * | 1989-07-05 | 1991-01-09 | Canon Kabushiki Kaisha | Light detecting device and light detection method |
EP0407166A3 (en) * | 1989-07-05 | 1991-08-07 | Canon Kabushiki Kaisha | Light detecting device and light detection method |
US5155093A (en) * | 1989-07-05 | 1992-10-13 | Canon Kabushiki Kaisha | Light detecting device and light detecting method using a superconnector |
EP0590738A1 (en) * | 1989-07-05 | 1994-04-06 | Canon Kabushiki Kaisha | Light detecting device and light detecting method using a superconductor |
US5296721A (en) * | 1992-07-31 | 1994-03-22 | Hughes Aircraft Company | Strained interband resonant tunneling negative resistance diode |
US6365912B1 (en) | 1998-06-17 | 2002-04-02 | Isis Innovation Limited | Superconducting tunnel junction device |
Also Published As
Publication number | Publication date |
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DE3175179D1 (en) | 1986-09-25 |
EP0081007A1 (en) | 1983-06-15 |
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